The present invention relates to a method for producing sheet material of a desired weight per unit area and more particularly to a method for efficiently changing from one weight to another.
The sheet material to which the present invention is particularly directed is tire fabric. Such fabric may be made by calendering rubber onto a textile cord or onto a steel cord. Final basis weight or weight per unit area of the tire fabric is controlled by adjusting the gaps of the calender rolls. A feedback control system for accomplishing the foregoing is disclosed and claimed in U.S. Pat. No. 3,889,121 entitled "Apparatus For Measuring The Weight Per Unit Area of a Manufactured Sheet Product Consisting of a Reinforcing Material Surrounded by a Bulk Material", in the name of David A. Bossen and assigned to the present assignee.
Generally in the production of tire fabric each sheet of rubber which is calendered onto the steel or fabric cord of a particular type is given a particular code reference. The code reference is, of course, directly related to the final weight per unit area of the tire fabric. By long practice in the industry each code is related to a nominal gap in the calender rollers which will theoretically produce a tire fabric close to the desired weight per unit area. With a particular gap setting the final tire fabric produced will still vary depending on the temperature and speed of the calender rollers and the density and temperature of the rubber itself. In the past the nominal gap setting was chosen with these variables in mind and through empirical experience. However, only a very approximate nominal setting was possible and even then all variables could not be taken into consideration. For example, depending on the type of rubber being processed the separation force, which might be as much as 250,000 pounds, caused a bend of the calender rollers or a deflection of the entire frame of the calenders to cause errors in the basis weight of the final product.
Thus in one type of prior art operation which did not use computer control, after the new nominal gap setting was accomplished by, for example, actuating the screw-down motor, a check of the basis weight of the tire fabric was made by actually cutting out a sample of the produced fabric. The measurement of the sample took approximately 31/2 minutes and the gap would be readjusted to produce a material of the proper weight per unit area. Because of the excessive time taken by this off-line test it is impractical where the entire running time of a particular "code" may be only seven mintues.
With computer control a more accurate measurement of the gap was sought by using a linear voltage displacement transducer (LVDT) on the rollers of the calender. For example, the feedback control technique of the Bossen patent could be used with a LVDT. However, the LVDT in measuring the gap at a position other than where the sheet is being formed again did not perform adequately.
There are two problems in computer feedback control. First, the nominal setting of the gap, when a new code is being run, initially will produce a sheet material which may have a basis weight vastly different than the desired basis weight. If the feedback control system is adjusted to bring the basis weight on target the feedback system will have an inherently slow and defective response; in other words, it will be incapable of accurate control with a low percentage of error from the desired target weight. On the other hand, a feedback control system tuned to work for only small differences from target will not adequately function when a large difference exists.
Another difficulty is the fact that there is an extremely complex and nonlinear relationship between the various parameters such as the bending of the calender rollers and the desired weight per unit area or code. For example, temperature variation changes the viscosity of the rubber material to thus change the separation force in the gap between the rollers thus changing the gap. In addition, the nominal gaps associated with each code have been notoriously unreliable varying in practice from 2 or 3 times from the stated nominal value.